The Prior Art
Carbon blacks have heretofore been incorporated into millable (as distinguished from flowable precursors) polyurethane rubbers, and have been found to reinforce them to a degree. However, when the types of carbon black ordinarily used for the generality of rubbers are incorporated in any substantial amounts into low-molecular-weight flowable and castable polyurethane rubber precursors, and also when porous carbon blacks are so incorporated without subjecting the mixture to vacuum treatment as disclosed hereinbelow, the flowability is greatly impaired, and the physical properties of the final cured rubber products are very little, if at all improved, see Axelwood et al., Rubber Age, 96 (2) 2233 (1964). In fact, if a sufficient quantity of carbon black is added to significantly affect the properties of the cured rubber products, the precursors become so viscous that they cannot, as a practical matter, be formed by casting, flowing, foaming, injection or any other procedure dependent upon flowability.